Questions① What determines stable upright posture?② What are the vestibular system contributions to posture?③ What are the somatosensory contributions to posture?④ What are the visual contributions to posture?⑤ What is the role of the brainstem in postural control?⑥ How do the muscle respond to postural disturbances?POSTURAL CONTROLPOSTURAL ORIENTATION(posture)POSTURAL STABILITY(equilibrium/balance)• Alignment of body segments• Body’s orientation in the environment• Control of centre of mass (COM) relative to base of support (BOS)Postural muscles and the ideal postural configurationIf an object falls over or not is entirely a matter of the vertical projection of its center of gravity relative to its supports. If this line of projection (X) lies within the area defined by the points of contact with the ground (the support area), the object does not fall (left, center). If X lies outside this critical area, then the system is unstable, and any further tilting will cause an ever-increasing rotational force that will make the object fall (right). Red arrows represent the differences of pressure at different points of support.Postural stabilityThe man clearly has an excellent upright posture, but is equally clearly about to experience a postural disaster, because the vertical projection of his center of gravity lies outside his region of contact with his support.Questions① What determines stable upright posture?② What are the vestibular system contributions to posture?③ What are the somatosensory contributions to posture?④ What are the visual contributions to posture?⑤ What is the role of the brainstem in postural control?⑥ How do the muscle respond to postural disturbances?Vestibular contribution to Control of upright stanceOtolith receptors• Static response and population coding = head orientation• Phasic response = linear acceleration/decelerationSemicircular canals• Phasic response = angular acceleration/deceleration• Left versus right response = direction of head movementVOR – coordinates eye-head movementsOtolith receptors detect head orientation and linear accelerations• Axis of mechanical sensitivity of each hair cell in the utricle is oriented toward the striola• Results in variation in axes (arrows) in population of hair cells• Therefore, tilt in any direction depolarizes some cells, hyperpolarizes some cells, and has no effect on some cells• Utricle and sacculus combined track linear forces acting on the head in any directionOtolith receptors are stimulated by head orientation and linear acceleration• When otolith organ is tilted, force of gravity pulling on the otoconia causes them to move relative to the macular surface because otolithic membrane deforms• when head accelerates, utricle (and hair cell bodies) accelerate but otoconia tend to stay where they are so are displaced relative to the maculaTonic postural vestibular reflexes. If an animal is suspended in the air, and its head and body tilted nose downwards, extension of the front legs and retraction of the rear can be observed.Vestibular system responsible = otolithsStatic contributionsSemicircular canalsSensory receptors: hair cells in the ampullaSense head rotationsReceptors in in the ampula are stimulated by angular accelerationsPaired canals work together to signal head movement• Because of inertia, rotation of the head in CCW direction causes endolymph to move clockwise with respect to the canals• Causes depolarization on the left• In the right canal, hair cells are hyperpolarized and afferent firing decreasesFunctional organization of the semicircular canals Paired canals work together to signal head movementPerpendicular canal arrangement for movement coding• Receptors are stimulated by angular accelerations of the head about different axes of rotation• Horizontal canal receptors – head rotation about vertical axis• Anterior & posterior canals – two perpendicular axes that lie in horizontal planeVestibulo-ocular reflex (VOR)• Compensates for head movement• Vestibular system signals how fast head is moving and oculomotor system uses this information to stabilize the eyes• Head movement to left, eyes deviate to right (vice-versa)• Often need to suppress VOR when it is not neededVestibular nystagmus. Rotating chair: Head motion results in smooth counter-rotation of the eyes in opposite direction to head motion (slow phase), interrupted at more or less regular intervals by a quick flick in the other direction (quick phase), giving rise to a sawtooth-like eye movement. Initiated by semicircular canalsDynamic contributionsConnections underlying the vestibulo-ocular reflex 1. Head turns left2. Eyes turn right3. Activate: left medial rectus and right lateral rectusGalvanic Vestibular Stimulation (GVS)Application of weak electrical stimulation to mastoid bones that carries through the head and interferes with vestibular system. Leads to postural sway responses. When seated leads to illusion of moving!!GVS elicits sway in the direction opposite to the negative electrode (the side the person perceives they are tilting to). This response is automatic and involuntary.https://youtu.be/Px6sOkjj-ZE?t=2m35sQuestions① What determines stable upright posture?② What are the vestibular system contributions to posture?③ What are the somatosensory contributions to posture?④ What are the visual contributions to posture?⑤ What is the role of the brainstem in postural control?⑥ How do the muscle respond to postural disturbances?Extensor thrust: on pushing up on the sole of one of its feet, the animal responds by extending the corresponding limb.Somatosensory contributions to postural control• Pressure receptors in the feet pressure at different points of contactLoss of postural stability: associated with increased Center of pressure (CoP) or Center of Mass (CoM) excursion or variabilitycenter of massAnnouncements November 30Team project #4: Traumatic Brain Injury (TBI)/Concussion – Case StudiesReadings: • ”Mild Traumatic brain injury: Neuroimaging of Sports-related concussion” (Mendez) • “Consensus statement on concussion in sport: the 4th International Conference on Concussion in Sport held in Zurich, November 2012” (McCrory et al.)• “A Brief Vestibular/Ocular Motor Screening (VOMS) Assessment to Evaluate Concussions” (Mucha et al.)• VOMS and GCS score sheets/insturctionsReviewLoss of postural stability: associated